1. Field of the Invention
The present invention relates to an information recording medium for writing (i.e., recording) information by means of laser beam.
2. Description of Prior Art
Information recording media for recording and/or reproducing information by the use of laser beam have been developed in recent years and are put to practical use. Such recording media have been widely utilized in various fields, for example, as an optical disc such as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file, micro-image recording medium, ultramicro-image recording medium, micro-facsimile, and optical card.
The conventional optical information recording medium basically comprises a transparent substrate of a plastic or glass material and a recording layer provided on the substrate, and the recording layer is generally composed of a metal such as Bi, Sn, In or Te, and a semi-metal. Between the substrate and the recording layer, an undercoating layer or an intermediate layer made of a polymer material is generally provided from the viewpoints of improving surface smoothness of the substrate and adhesion between the substrate and the recording layer and/or increasing sensitivity of the resulting medium. Writing (i.e., recording) of information on the recording medium can be conducted, for example, by irradiating the medium with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the recording medium absorbs energy of the beam and rise in temperature locally occurs, and as a result a chemical or physical change is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby the recording of information can be made. Reading of the information from the recording medium is also conducted by irradiating the medium with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the change in the optical characteristics of the recording layer.
Recently, an optical disc having an air-sandwich structure for protecting the recording layer has been proposed. The optical disc of air-sandwich structure comprises two disc-shaped substrates, a recording layer provided on at least one of the substrates and two ring-shaped spacers (inner spacer and outer spacer), said two substrates interposing the recording layer being combined with each other in such a manner that a closed space is formed by the two substrates and the two spacers. In such recording medium, the recording layer is kept from direct exposure to an outer air, and recording or reproduction of information is carried out by applying light of the laser beam to the recording layer through the substrate, whereby the recording layer is generally kept from physical or chemical damage. Further, the surface of the recording layer can be kept from deposition of dust which likely causes troubles in the recording or reproducing procedure.
In the above-mentioned conventional information recording medium, the recording layer made of a metal or a semi-metal can be generally formed on the substrate of plastic or glass material by a method such as a deposition (metallizing) method or a sputtering method.
The deposition method is advantageous for providing a recording layer, but has a problem of preservability as far as a material such as Te having high sensitivity to laser and capable of forming a film by deposition is concerned. Further, the method needs an expensive vacuum apparatus and high vacuum technique, and furthermore the method requires a long period of time for forming a recording layer.
The sputtering method is more advantageous than the deposition method in the formation of a recording layer, but it is difficult to precisely determine the thickness of the recording layer and an expensive apparatus is required in the method.
An information recording medium using a dye as a recording material is also known. In this case, a dye is generally dissolved or dispersed in an appropriate solvent to prepare a solution, and the solution is coated over a substrate to form a recording layer on the substrate. This coating method using the coating solution to form a recording layer is excellent in mass production and provides a recording medium at low cost. However, the recording layer prepared by the coating method is inferior to the above-mentioned recording layer made of metal or semi-metal by the deposition or sputtering method in resistance to light or heat.
Accordingly, an information recording medium having the conventional recording layer formed by the deposition method, sputtering method or coating method is desired to be improved in various properties such as endurance, mass productivity and recording sensitivity.